'Run Fat Boy Run' a workout

I was looking forward to seeing "Run Fat Boy Run," and not just because I can relate to half of the title.

The main reason: It stars (and was co-written by) Simon Pegg, the lead in two of the funniest movies of the past few years, "Shaun of the Dead" and "Hot Fuzz." Pegg is great at playing characters who are knuckleheads yet are believably human rather than caricatures.

And though I had my doubts about "Fat Boy" because it was directed by David Schwimmer, whom I never found funny on "Friends" or anywhere else, my general optimism about the comedy was rewarded.

It had me busting out laughing.

Pegg plays Dennis, a Londoner who, as the film opens, experiences a case of the jitters and bolts from his wedding, leaving his pregnant bride-to-be, Libby (Thandie Newton), at the altar.

Five years pass. Dennis is a clothing-shop security guard. He's still in contact with Libby, who allows him to spend time with their son, Jake (Matthew Fenton). But now there's a new man in Libby's life, American businessman Whit (Hank Azaria), who seems a little too perfect to be true and is. Dennis suffers for his wedding-day weakness; he wishes he could have Libby back. But how do you compete with Mr. (Seemingly) Perfect?

It just happens that Whit is a runner and there's a huge charity marathon about to be run through the streets of London. Dennis decides that, though he's a smoker with a paunch, and the idea of doing any exercise is almost beyond his comprehension, he will prove to Libby his ability to follow through on something by training for and completing the marathon himself.

You can probably see where all of this is going, and it's OK if you do. This is not a movie with a lot of plot surprises. The appeal of the film is what you get in the context of the familiar — whether it's seeing Dennis suffer through a spinning class (hilarious) or simply hearing him react to Whit telling him he runs long distances by asking, incredulously, "Why would you do that?"

What is surprising is how it actually works as a romantic comedy with some warmth. You'd never guess this from the opening sequence, when Dennis makes his cowardly departure from his wedding. How can you possibly root for such a weasel?

And, yet, we do. Dennis represents anyone who has been a jerk in the past and longs for a chance at redemption. Watching Dennis put himself through hell to win back Libby, I was reminded of Jack Nicholson's great line to Helen Hunt in "As Good as It Gets" when he tells her, "You make me want to be a better man." Something on that order is going on here, and it's actually moving.

And really, really funny.

Tim Miller is the Times' entertainment editor. He can be reached at tmiller@capecodonline.com.